The present invention relates to a method of copy protecting an optical disc and to a copy protected optical disc. In addition, the application relates to a method for encoding user data and a method for selecting data symbols for incorporation within user data.
Optical discs, such as the various formats of compact discs (CDs) and of digital versatile discs (DVDs) are increasingly used for carrying information for many different applications. The information encoded onto the optical disc is generally very valuable and, accordingly, they are increasingly copied by counterfeiters. Furthermore, recordable CDs and CD writers for writing the information content from one disc to such recordable discs are readily available to the domestic consumer. Recordable DVDs and DVD writers have become as readily available. This means that new and effective methods for copy protecting optical discs are required.
The applicants have proposed various copy protection techniques which utilise data patterns which have poor Digital Sum Value (“DSV”) characteristics. For example, in WO 02/11136 data patterns are added to a CD to provide an authenticating signature. These data patterns are chosen to cause DSV problems. It has been found that when a CD writer is used to make a copy of the original disc it has difficulty writing the authenticating signature.
In PCT/GB2004/000241 areas of unbalanced dc content are added to an optical disc by recording onto the disc data with poor DSV characteristics. It has been found that if the areas of recorded data with unbalanced dc content are restricted in size, for example, there is no problem in playing the disc normally but again, copying of the disc is made very difficult.
It will be seen from the above that it is very useful to impress data patterns with poor DSV characteristics onto optical discs.